Many types of electrical equipment require a human user to provide data to them for controlling their operation. Where the amount of control data is relatively small, the practice is often to provide a keypad which a human can manipulate to provide the needed control data. There are any number of examples of such keypads in our daily life, with those on telephones and calculators being perhaps the most common. These keypads are for the most part left to be accessible at all times. Keypads which are subjected to heavy use, such as those in calculators and telephones are usually of the button type. Another type of keypad such as is often found on microwave ovens, is of the membrane type where an area of a flexible membrane forms the operating surface of one key, and the switch is formed beneath the operating surface.
There are other situations, typically where there is only occasional need to access the keypad, where a cover is provided for the keypad. A cover may be provided perhaps for reasons of protecting the keypad, for controlling access to it, or simply for esthetics. Where a cover is provided, it may be desirable to allow access to one or more of the keys without moving the cover to expose the entire keypad. One situation may be to provide such an external access for the on/off switch. In another, the occupant may wish access to the set point control keys of an electronic thermostat without having to lift the cover, or in the dark, without having to turn on a light.
To accomplish this, it is convenient to provide an external pushbutton which is part of a pushrod accessible from outside the cover, and which can operate a key beneath the cover. Such a pushrod is supported in some way within an aperture or hole in the cover, where the aperture is in alignment with the key which is to be operated with the cover over the keypad. There are such pushrods at the present time which are supported in their aperture by a resilient cantilever support arm which is fastened to the interior surface of the cover. Pushing on the pushrod deflects the support arm and operates the key beneath the end of the pushrod. The resiliency of the support arm is arranged to hold the pushrod away from the key surface when the pushrod is not actively being pressed by a user.
There are a number of potential problem with these cantilever support arms. One is that operating the pushbutton creates an appreciable amount of tensile stress on the support arm's attachment point closer to the pushbutton. Since these housings are now usually made from some kind of thermoplastic or thermosetting plastic by an injection molding process, the material for the cover, the support arm, and the attachment between them are all preferably made from such a material. Such material is not renowned for its tensile strength, so the tensile stress which operating the pushbutton creates can cause failure of the support arm attachment to the interior of the cover.
The injection molding process is a cheap and reliable way of creating plastic parts with accurate dimensions. One rule of thumb when using this process is to limit the number of parts required for a complete unit of whatever device is involved. Another rule of thumb is to simplify the assembly process as much as possible. One common assembly expedient is to mold in the various parts, posts and holes into which the posts mate. Then after mating the posts with their corresponding holes, the projecting ends of the posts are staked to complete the step of attaching one part to another. Staked post assembly creates a final product whose parts are very strongly attached to each other when subjected to compressive and shear loads, but not so strongly attached when in tension, where the staked material provides a relatively small area in shear to resist tensile load.
These various competing engineering considerations may sometimes result in trade-offs in a design. All of these considerations are involved in designing a cover to be produced by injection molding and having external pushbuttons for operating a key of the keypad which it covers.